This invention relates generally to nanoscale fibers such as carbon nanotubes and nanofibers, and more particularly to actuators comprising carbon nanotubes and/or nanofiber films.
Actuation in the open air under atmospheric conditions has previously been explored with electroactive polymer (EAP) actuators. These EAP acutators have a low operational voltage and large deformation. EAP actuators were usually operated in liquid electrolyte solutions until 2003, when Zhou et al. first developed a solid state composite actuator that worked in air. That actuator was based on polypyrrole and polymer-in ionic liquid electrolytes. However, EAP actuators have some intrinsic disadvantages, including low mechanical strength, small force output, large driving currents, poor repeatability, and poor durability under ambient and open air conditions. These disadvantages are a result of the EAP actuation mechanism being based on the Faradaically driven redox reaction of the conductive polymers. Recently, other EAP actuators actuating under ambient and open air conditions have been reported. However, their sample preparation process not only required multiple procedures, such as conductive polymer film preparation and metal layer deposition, but each procedure was also undesirably time consuming.
Films of carbon nanotubes and nanofibers, or buckypapers, are a potentially important material platform for many applications. Typically, the films are thin, preformed sheets of well-controlled and dispersed porous networks of single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), carbon nanofibers (CNFs), or mixtures thereof. The carbon nanotube and nanofiber film materials are flexible, light weight, and have mechanical, conductivity, and corrosion resistance properties desirable for numerous applications. The film form also makes nanoscale materials and their properties transferable to a macroscale material for ease of handling.
Due to the exceptional properties of carbon nanotubes, their use in high performance electromechanical actuators has been explored. A nanotube actuation mechanism may rely on quantum chemical expansion of a graphitic carbon lattice when an electrical charge is applied to the nanotube. In effect, electrochemically charging and discharging carbon nanotubes (CNTs) can generate motion. Such actuation was reported by Dr. Ray Baughman for a SWNT buckypaper actuation device in an aqueous electrolyte (Science, 284, 1340, 1999). If instead of liquid electrolytes, solid electrolytes are used to realize in-air or “dry” actuation, numerous applications, such as use in composite morphing structures for aircraft, exist for nanotube actuators.
Other research by Baughman et al., (Advanced Materials 18:870-873, 2006) also focused on SWNTs as an actuator material. Carbon nanotube actuators have good mechanical properties, a wide potential window in electrochemical reaction, large surface areas, and superior conductivity to enhance actuation performance. The actuation mechanism of these actuators was based on quantum chemical expansion of carbon-carbon bonds due to electrochemical double layer charging and discharging of CNTs. The advantage of SWNT actuator systems is that they directly convert electrical energy to mechanical energy. Thus, Baughman et al. used repulsive force to trigger CNT film actuation via charge injection with applied ultra-high electric voltage. The CNT films were the sole component of artificial muscles that provided elongations of 220% and worked at elevated temperature.
Concurrently, Aida et al. (Advanced Materials 20:1-4, 2009) developed high conductive sheet actuators which included long CNTs mixed with ionic liquids. The sheet fabrication took three days. The mechanical properties of Young's modulus and strength of the sheets were relatively low, at 156±59 MPa and 17±4 MPa, respectively. At an applied frequency of 1 Hz, a displacement as large as 5 mm was observed.
In addition, a custom-made solid CNF/Poly Methylmethacrylate (PMMA) electrolyte actuator was reported by a research group at The University of Cincinnati (Composites: Part B, 37:382-394, 2006). Briefly, the group reported using a combination of solvent casting and melt mixing to make the CNF/PMMA composites. A solid polymer electrolyte (SPE) film was prepared from PMMA, lithium tetrafluoroborate (LiBF4), propylene carbonate (PC), and acetronitrile (ACN) dissolved at 70° C. without any purification. The solution was stirred continuously until the mixture became a homogeneous viscous liquid. Then this solution was poured into a mold to cast the SPE film. The SPE film was dried in a vacuum oven for 2 hours. The resultant SPE was 100-500 μm thick and was peeled off the mold and cut into 0.7 cm width×2 cm length films. Due to relatively low electrical conductivity (i.e., in the range of 10−3 S/cm of the actuators), the actuation performance of the actuators including the SPE film was relatively low as compared to the wet CNF/PMMA actuators.
There have also been several efforts to use NAFION® (a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer) as a SPE for nanotube actuators. DuPont NATION®, such as NAFION® NRE-212 membrane, is a commercial, high conducting SPE material which can be used in fuel cell and battery applications. NAFION® is highly conductive to cations, making it useful for nanotube actuation applications. For instance, NAFION® NRE-212 membrane is a 2 millimeter thick, highly conductive proton exchange conductor. NAFION® conductivity is in the range of 10−2 S/cm, which is two orders of magnitude higher than ionic liquid, and ten orders of magnitude higher than undoped conjugated polymers. In addition, NAFION® conductivity is affected by humidity in the range of 10−2 S/cm.
SWNT/NAFION® composite actuators have been made by a research group at Rochester Institute of Technology (Materials Science and Engineering B: Solid-State Materials for Advanced Technology, v 116, n 3 SPEC.ISS., 359-362, 2005). In that research, purified SWNTs were dispersed in a NAFION® solution to cast SWNT/NAFION® composite actuators. These bimorph actuators were made with an insulating substrate as shown in FIG. 1. However, the actuators needed to be in a liquid electrode for actuation. In addition, the actuation performance was relatively low due to low nanotube concentration and low electrical conductivity of the samples as compared to buckypaper-based actuators. The same research group also used SWNT buckypaper infiltrated with a NAFION® solution to prepare actuators (Proc SPIE Int Soc Opt Eng, v 4695, 52-56, 2002). However, their process resulted in a substantially exfoliated layer morphology that caused a reduction in both conductivity and actuation strain (about 0.03%).
It therefore would be desirable to provide improvements in actuators having nanotubes and/or nanofiber films and to provide methods for making improved actuators having nanotube and/or nanofiber films. For example, it would be desirable to provide actuator structures that reduce or avoid the aforementioned deficiencies. It also would be desirable to provide improved methods for producing actuators that include nanotubes and/or nanofiber films.